The present invention relates to a process for the production of an integrated laser-photodetector structure.
The technical field of the invention is that of double heterostructure semiconductor lasers either of the conventional type, or of the quantum type (the latter generally being called "multiple quantum well") and with a ribbon-like junction geometry. A double heterostructure is constituted by a stack or pile of different semiconductor alloy films. The latter are deposited on a monocrystalline substrate, either by liquid or gaseous phase epitaxy, or by a molecular beam. The thickness of the active zone, where both the excitons and photons are produced, is approximately 100 nm for a conventional double heterostructure. It is reduced to about ten nanometers in the case of a quantum structure.
For lasers emitting between 0.8 and 0.9 .mu.m, the films constituting the double heterostructure are of alloy Ga.sub.1-x Al.sub.x As, whilst for lasers emitting between 1.3 and 1.65 .mu. the films are of alloy Ga.sub.1-x In.sub.x As.sub.1-y P.sub.y.
In optical fiber telecommunications, which is the favoured field for semiconductor lasers, the power emitted by the lasers is sensitive to temperature fluctuations and to partial deterioration in the case of long operating periods. Therefore power stabilization of the laser emitter is necessary. This stabilization is obtained by a photodetector positioned behind the laser. The detected signal is injected into a feedback circuit making it possible to readjust the supply current of the laser. This detector which, like the laser, is mounted in the optical head must be accurately positioned in order to collect the maximum power. During its fitting, it may also be necessary to incline it in order to prevent instability of the emission by the reinjection of photons into the laser cavity.
Of late, numerous works have appeared on the integration of a laser and a photodetector. Two solutions have already been reported, namely monolithic integration and hybrid integration.
1. In the first of these methods, it is a question of producing a channel perpendicular to the ribbon, tape or strip of the laser by chemical or ionic etching of the double heterostructure, which makes it possible to separate the two components, namely the laser on one side and the photodetector on the other. The functions of these two components are differentiated by the polarity of the voltage applied to the p-n junction which they both have. The laser junction is polarized in the conductive direction (direct p-n), whilst the photodetector junction is polarized in the reverse direction. Two integration types are possible as a function of the coupling mode between the laser and the photodetector.
(a) Coupling Across a Guide
In this case, the double heterostructure is constituted on the one hand by the different conventional layers or films (first confinement layer, active layer, second confinement layer and contact layer) and on the other hand by a passive guide located beneath the first confinement layer. Its index has an intermediate value between that of the active zone and that of the confinement layer. Etching of the different layers is then stopped just below the active zone in the first confinement layer. Coupling between the two components takes place across the guide and is dependent on the thickness of the first confinement layer, as well as the profile of the opening produced. Such a coupling mode is described in the article by J. L. MERZ and L. R. LOGAN entitled "Integrated GaAs-GaAlAs injection laser and detector with etched reflector" published in Applied Physics Letters, vol. 30, No. 10, p 530, 15.5.1977 and in the article by P. D. WRIGHT, R. J. NELSON AND R. B. WILSON entitled "Monolithic Integration of InGaAsP Heterostructure Lasers and Electrooptical Devices" published in IEEE Journal of Quantum Electronics, vol. QE 18, No. 2, February 1982.
(b) Direct Coupling
The double heterostructure does not necessarily have a passive guide beneath the active layer. The opening made is sufficiently deep to eliminate any guided coupling between the laser and the detector. Two active layer sections then face one another, namely one for transmission and the other for detection. A device of this type is described in the article by O. WADA et al entitled "AlGaAs-GaAs Microcleaved Facet (MCF) Laser Monolithically integrated with photodiode", published in Electronic Letters, vol. 18, No. 5, 4.3.1982 and in the article by Yoshia Suzuki et al entitled "InP/InGals 1.5 .mu.m Region etching cavity laser" Jpn. J. Appl. Phys., vol. 23, 1984.
2. The second method is that of hybrid integration. Such a solution is described in U.S. Pat. No. 4,297,653 "Hybrid semiconductor Laser/detectors" granted to D. R. SCIFRES et al. In this procedure, the laser is mounted on a machined, diffused silicon support for providing a detector in front of which is prepositioned the laser. Thus, a rigid laser-photodetector assembly is obtained.
Although they are satisfactory in certain respects, these methods suffer from certain disadvantages. In general terms, integrated laser-photodetector structures are mounted on the substrate side on a heat dissipator for reducing their operating temperature. The by no means negligible thermal resistance of these lasers assumes that they have a low threshold current in order to permit a continuous operation. Moreover, a good sensitivity of the feedback system (ratio for the variation of the photodetected current to that of the supply current of the laser) is desirable.
In the case of the solution using a passive guide, the threshold current is doubled compared with an insulated laser having two cleaved faces (6 kA/cm.sup.2 instead of 3 kA/cm.sup.2). This structure also requires a very precise control of the etching of the mirrors so as to ensure that the optical guide is not etched. Authors who have reported on this method do not describe continuous characteristics. The sensitivity values measured are approximately 10%.
The integrated structure with direct coupling makes it possible to obtain relatively low threshold currents (approximately 40 mA) essentially due to the quality of the microcleaved faces. However, the sensitivity value remains low (approximately 1.5%), which is due to the low active detection surface (active layer thickness approximately 10 nm). The sensitivity is even lower in the case of double heterostructures with quantum layers or films (thickness approximately 10 mm) and sensitivity of approximately 0.045%.
The laser-photodetector hybrid structure provides the desired performance characteristics of good quality of the mirror faces of the lasers (cleaved faces) and high sensitivity of the detector (silicon detector). The limitations of this procedure relate to the hybridization causing problems of precision, reliability and fitting costs.